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Abstract:

A radiation crosslinked (50 kGy), pressure-treated UHMWPE material has
been developed by applying compressive force on a crosslinked polymer in
a direction orthogonal to an axial direction. The deformed material is
then cooled while held in a deformed state. The resulting material is
anisotropic, with enhanced strength oriented along the axial direction.
The directionally engineered material is oxidatively stable even after
four weeks of accelerated aging in a pressure vessel containing five
atmospheres of oxygen (ASTM F2003). Because of its oxidative stability,
the deformation processed material is a suitable candidate for
air-permeable packaging and gas sterilization, which has thus far been
reserved for remelted highly crosslinked UHMWPEs.

Claims:

1. A method for treating a polymeric bulk material comprisingheating a
cross-linked polymer to a compression deformable temperature, the polymer
being in an elongated bulk form characterized by an axial direction in
the direction of the elongation;applying force to deform the heated
polymer in a direction orthogonal to the axial direction; andcooling the
polymer to a solidification temperature while maintaining the polymer in
a deformed state.

2. A method according to claim 1, wherein the compression deformable
temperature is less than the melting point of the polymer and greater
than the melting point minus 50.degree. C.

3. A method according to claim 1, wherein the crosslinked polymer is in
the form of a cylindrical rod.

4. A method according to claim 1, wherein applying force comprises
reducing a dimension of the bulk material in the orthogonal direction.

5. A method according to claim 1, comprising extruding the heated polymer
through a reducing die.

6. A method according to claim 1, wherein the polymer is crosslinked with
gamma-irradiation.

7. A method according to claim 1, wherein the polymer is ultrahigh
molecular weight polyethylene (UHMWPE).

8. A method according to claim 7, wherein the melt deformation temperature
is less than the melting point and greater than the melting point minus
50.degree. C.

9. A method according to claim 7, where in the UHMWPE is in the form of a
cylindrical rod.

10. A method according to claim 7, wherein the UHMWPE is crosslinked with
gamma-irradiation.

11. A method according to claim 7, wherein the UHMWPE is crosslinked with
from 0.01 to 10 Mrad of gamma-irradiation.

12. A method according to claim 7, comprising extruding the heated UHMWPE
through a reducing die.

13. A method according to claim 7, comprising cooling the deformed polymer
to a temperature below 30.degree. C. to solidify it.

14. A method according to claim 7, further comprising stress relieving the
solidified polymer by heating to a temperature above about 100.degree. C.
and below the melting point.

15. A method according to claim 14, wherein stress relieving is carried
out at 125-135.degree. C.

16. A method for making an implant bearing component, comprising machining
the component from a UHMWPE prepared according to claim 14.

17. An implant, comprising UHMWPE prepared according to claim 14.

18. A method for treating crosslinked UHMWPE to make material suitable for
use in medical implants comprisingheating UHMWPE to a temperature above
about 80.degree. C. and below its melting point, wherein the UHMWPE has
been crosslinked with gamma-irradiation and is in the form of a bulk
material characterized by an axial direction, a transverse direction
orthogonal to the axial direction and an original transverse
dimension;applying compressive force to reduce a dimension of the bulk
material in the transverse direction;cooling the bulk UHMWPE to a
solidification temperature while maintaining compressive force sufficient
to prevent the bulk material from returning to its original transverse
dimension.

19. A method according to claim 18, wherein applying compressive force
comprises extruding the bulk material through a reducing die.

20. A method according to claim 18, comprising extruding the heated
crosslinked bulk material through a reducing die into a chamber
dimensioned to hold the bulk material at a dimension in the transverse
direction less than its original transverse dimension.

21. A method according to claim 18, comprising heating the bulk
crosslinked UHMWPE greater than 100.degree. C., extruding through a
reducing die, and cooling the extruded UHMWPE to below 30.degree. C. in a
cooling chamber.

22. A method according to claim 18, comprising heating the bulk UHMWPE to
about 130.degree. C., extruding the heated UHMWPE through a reducing die
into a cooling chamber, and holding the extruded bulk material in the
cooling chamber until the temperature of the cooling chamber drops to
30.degree. C.

24. A method according to claim 23, wherein the extruded cooled bulk
UHMWPE is held straight in a mechanical device during the stress
relieving.

25. A method according to claim 23, wherein the stress relief heating is
at 120.degree. C. or higher.

26. A method according to claim 23, wherein the stress relief heating is
at about 125.degree. C. to 135.degree. C.

27. An implant, comprising UHMWPE treated according to claim 23.

28-44. (canceled)

45. A method of making a bearing component made of UHMWPE, suitable for
use in a medical implant, comprising:radiation crosslinking a
UHMWPE;preheating the crosslinked UHMWPE to a temperature above
80.degree. C. and below its melting point;solid state extruding the
UHMWPE at a draw ratio of greater than 1;cooling the extruded UHMWPE to a
solidification temperature below 80.degree. C. while maintaining
diametral compression;annealing the cooled UHMWPE at a temperature below
the melting point for a time sufficient for the rod to increase in
diameter in response to the annealing; andmachining the component from
the annealed UHMWPE.

46. A method according to claim 45, comprising crosslinking to a dose of
0.01 to 10 MRad, heating to a compression deformable temperature of
125-135.degree. C., extruding with a draw ration of 1.2 to 1.8, cooling
to a solidification temperature of 30.degree. C., and stress relieving at
12-135.degree. C.

47. An implant comprising a bearing component made by a process according
to claim 45.

48. An implant comprising a bearing component made by a process according
to claim 46.

49-67. (canceled)

Description:

CROSS REFERENCE TO RELATED APPLICATIONS

[0001]This application claims the benefit of a U.S. Provisional
Application filed by David W. Schroeder et al. on Oct. 7, 2004, entitled
"Crosslinked Polymeric Material with Enhanced Strength and Process for
Manufacturing", the entire disclosure of which is hereby incorporated by
reference.

[0002]The invention relates to crosslinked high molecular weight polymeric
material and methods for treating the materials to provide enhanced
properties. In particular, the invention provides methods and materials
for use in preparing polymeric implants with a high degree of wear and
oxidation resistance.

[0003]Crosslinked ultra high molecular weight polyethylene (UHMWPE) is now
widely used in medical implants such as acetabular components for total
hip replacements. There remains interest by the orthopedic community to
find alternative methods of processing radiation crosslinked UHMWPE to
improve mechanical properties while still retaining wear resistance and
oxidative stability in the material.

[0004]In U.S. Pat. No. 6,168,626, Hyon et al. report enhancement of the
mechanical properties of crosslinked UHMWPE by deformation processing at
a compression deformable temperature. After deformation processing, the
material is cooled while keeping the deformed state. An oriented UHMWPE
molded article is obtained that has an orientation of crystal planes in a
direction parallel to the compression plane. The compression is carried
out using a suitable die or can be done using a hot press machine.

[0005]Polymeric materials such as UHMWPE can be crosslinked to provide
materials with superior wear properties, for example. The polymeric
materials may be chemically crosslinked or preferably crosslinked with
irradiation such as γ-irradiation. The action of
γ-irradiation on the polymer results in the formation of free
radicals within the bulk materials. The free radicals provide sites for
reactions to crosslink the molecular chains of the bulk materials. It has
become recognized that the presence of free radicals, including any free
radicals that survive after subsequent heat treatment, are also
susceptible to attack by oxygen to form oxidation products. The formation
of such oxidation products generally leads to deterioration of mechanical
properties.

[0006]To completely remove free radicals and provide polymeric materials
of high oxidative stability, it is common to heat treat the crosslinked
material above the crystalline melting point of the polymer. This has a
tendency to destroy or recombine all of the free radicals in the bulk
material. As a result, the crosslinked material is highly resistant to
oxidative degradation. However, some desirable mechanical properties are
lost during the melting step.

[0007]It would be desirable to provide materials such as crosslinked
UHMWPE that combine a high level of mechanical properties and a high
resistance to oxidative degradation.

SUMMARY

[0008]A method for treating a polymeric bulk material comprises heating a
cross-linked polymer to a compression deformable temperature. The polymer
is in a bulk form characterized by an axial direction. Force is then
applied to deform the heated polymer in a direction orthogonal to the
axial direction, and the polymer is cooled to a solidification
temperature while maintaining the polymer in a deformed state.

[0009]In another embodiment, a method for treating crosslinked UHMWPE to
make material suitable for use in medical implants comprises heating the
UHMWPE to a temperature above about 80° C. and below its melting
point, wherein the UHMWPE has been crosslinked with gamma-irradiation and
is in the form of a bulk material characterized by an axial direction, a
transverse direction orthogonal to the axial direction and an original
transverse dimension. Compressive force is then applied to reduce a
dimension of the bulk material in the transverse direction, followed by
cooling the bulk UHMWPE to a solidification temperature while maintaining
compressive force sufficient to prevent the bulk material from returning
to its original transverse dimension.

[0010]In another embodiment, a UHMWPE pre-form suitable for use in medical
implants is prepared by heating a gamma-crosslinked UHMWPE rod
characterized by a crystalline melting point and a diameter d1 to a
compression deformable temperature; then applying a compression force on
the crosslinked UHMWPE to reduce the rod's diameter to d2 less than
d1; then cooling the smaller diameter rod of UHMWPE to a
solidification temperature while maintaining compression force sufficient
to hold the diameter at a diameter of d3, wherein
d3<d1; and finally stress-relieving the cooled rod by
heating it to a temperature at which the rod expands to a diameter
d4, wherein d4>d3.

[0011]Products of the process include a compression deformed crosslinked
polymeric material suitable for use as bearing components such as
acetabular cups in medical implants such as those used in hip
replacements. In a preferred embodiment, ultrahigh molecular weight
polyethylene is provided that has a combination of a high tensile
strength at break and a high resistance to oxidative degradation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]The present invention will become more fully understood from the
detailed description and the accompanying drawings, wherein:

[0013]FIG. 1 illustrates the geometry of an extrusion process;

[0014]FIG. 2 shows various embodiments of extrusion apparatus and dies;
and

[0015]FIG. 3 illustrates an embodiment of an extrusion process.

DESCRIPTION

[0016]The headings (such as "Introduction" and "Summary,") used herein are
intended only for general organization of topics within the disclosure of
the invention, and are not intended to limit the disclosure of the
invention or any aspect thereof. In particular, subject matter disclosed
in the "Introduction" may include aspects of technology within the scope
of the invention, and may not constitute a recitation of prior art.
Subject matter disclosed in the "Summary" is not an exhaustive or
complete disclosure of the entire scope of the invention or any
embodiments thereof. Similarly, subpart headings in the Description are
given for convenience of the reader, and are not a representation that
information on the topic is to be found exclusively at the heading.

[0017]The description and specific examples, while indicating embodiments
of the invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention. Moreover, recitation of
multiple embodiments having stated features is not intended to exclude
other embodiments having additional features, or other embodiments
incorporating different combinations of the stated features. Specific
Examples are provided for illustrative purposes of how to make, use and
practice the compositions and methods of this invention and, unless
explicitly stated otherwise, are not intended to be a representation that
given embodiments of this invention have, or have not, been made or
tested.

[0018]As used herein, the words "preferred" and "preferably" refer to
embodiments of the invention that afford certain benefits, under certain
circumstances. However, other embodiments may also be preferred, under
the same or other circumstances. Furthermore, the recitation of one or
more preferred embodiments does not imply that other embodiments are not
useful, and is not intended to exclude other embodiments from the scope
of the invention.

[0019]As used herein, the word "include," and its variants, is intended to
be non-limiting, such that recitation of items in a list is not to the
exclusion of other like items that may also be useful in the materials,
compositions, devices, and methods of this invention.

[0020]In one embodiment, the invention provides a method for treating a
polymeric bulk material comprising heating a crosslinked polymer to a
compression deformable temperature, applying force to deform the heated
polymer, and cooling the polymer to a solidification temperature while
maintaining the polymer in a deformed state. The crosslinked polymer is
in a bulk form characterized by an axial direction; force is applied to
deform the heated polymer in a direction orthogonal to the axial
direction. Among other desirable physical properties, the polymeric bulk
material as treated by the above method exhibits enhanced strength in the
axial direction of the bulk material. When the bulk material is UHMWPE,
the method is especially suitable for providing a medical implant
containing bearing components made out of the UHMWPE.

[0021]In another embodiment, the invention provides a method for treating
crosslinked UHMWPE for making material suitable for use in a medical
implant. The method involves heating UHMWPE to a temperature above about
80° C. and below its melting point, where the UHMWPE has been
crosslinked with γ-irradiation. The UHMWPE is in the form of a bulk
material characterized by an axial direction, a transverse direction
orthogonal to the axial direction, and an original dimension. Compressive
force is then applied on the bulk material in the transverse direction to
the reduce a dimension of the bulk material in that direction. Then the
bulk UHMWPE is cooled to a solidification temperature. In one embodiment,
force is applied during the cooling sufficient to prevent the bulk
material from returning to its original dimension. In various
embodiments, compressive force is applied by ram extruding the bulk
material through a reducing die, for instance through a circular die,
with a diametral compression or draw ratio greater than 1.

[0022]In various embodiments, compressive force is maintained on the
cooling UHMWPE by extruding the heated crosslinked bulk material into a
chamber of sufficient size and shape to hold the bulk material at a
dimension in the transverse direction less than original radial dimension

[0023]In particular embodiments, the invention provides a method for
preparing a preform made of UHMWPE suitable for use in medical implants.
The method comprises heating a γ-irradiation crosslinked UHMWPE rod
characterized by a crystalline melting point and a diameter d1 to a
compression deformable temperature. Thereafter, compression force is
applied on the crosslinked UHMWPE to decrease the diameter to d2,
wherein d2 is less than d1. The reduced diameter rod of UHMWPE
is optionally cooled to a solidification temperature while maintaining
compression force to keep the diameter at a value of d3, wherein
d3 is less than d1. In a subsequent step, the cooled rod is
stress relieved by heating to a temperature at which the rod expands to a
diameter d4, wherein d4 is greater than d3. In various
embodiments, the method involves extruding the rod through a reducing die
into a cooling chamber to reduce the diameter from d1 to d2.
The compression deformable temperature is preferably less than the
melting point and greater than a temperature equal to the melting point
minus 50° C. In a preferred embodiment, the compression deformable
temperature is from about 100° C. to about 135° C.
Preferably, the UHMWPE rod has been crosslinked with γ-irradiation
at a dose from 0.1 to 10 Mrad. Methods are also provided for making a
bearing component for a medical implant from UHMWPE treated according to
the method above, as well as implants comprising a UHMWPE machined from a
preform made according to the above methods.

[0024]In one aspect, the invention provides a γ-crosslinked UHMWPE
in the form of an elongate material, such as a cylinder, characterized by
an axial direction. The tensile strength in the axial or longitudinal
direction is greater than 50 MPa and preferably greater than 60 MPa. In
preferred embodiments, bearing components comprise UHMWPE machined or
formed from such a γ-crosslinked UHMWPE. Medical implants contain
the bearing components.

[0025]In another aspect, the invention provides a γ-crosslinked
UHMWPE having a detectable concentration level of free radicals, but
nevertheless stable to oxidation as measured by standard tests. For
example, in a non-limiting example, the concentration of free radicals in
the UHMWPE is above about 0.06×1015 spins/g and below about
3×1015 spins/g. Preferably, the free radical concentration is
1.5×1015 spins/g or less. In a preferred embodiment, there is
no detectable increase in the carbonyl IR absorption band during exposure
to oxygen at 5 atmospheres for four weeks at 70° C. The
crosslinked UHMWPE is advantageously provided in the form of a
cylindrical rod having a diameter of about 2 to 4 inches and preferably
about 3 inches. Bearing components are provided by machining the
components from the crosslinked UHMWPE, and medical implants are provided
that contain the bearing components.

[0026]In a further aspect, an anisotropic crosslinked UHMWPE is provided
in the form of a bulk material characterized by an axial direction and a
transverse direction orthogonal to the axial direction. In various
embodiments, the anisotropy is characterized in that the tensile strength
in the axial direction is 20% or more greater than the tensile strength
in the radial direction, and attains the value of at least 50 MPa,
preferably at least 60 MPa.

[0027]In a further aspect, a method for solid state deformation processing
of γ-irradiated crosslinked UHMWPE comprises deforming the UHMWPE
by extruding it at a temperature below its melt transition or crystalline
melting point. In subsequent steps, the extruded UHMWPE is cooled to a
temperature below its solidification temperature, optionally while
maintaining the extruded rod in a deformed state.

[0028]In other embodiments, a compression deformed crosslinked UHMWPE
having a tensile strength at break of more than 50 MPa is provided by
treating UHMWPE according to the methods. In preferred embodiments, the
material is also resistant to oxidative degradation, characterized by an
oxidation index less than 0.5 after exposure to 5 atm of oxygen at
70° C. for 4 days, in spite of the material having a detectable
free radical concentration above 0.06×1015 spins/g.

[0029]In another aspect, the invention provides a method for making a
medical implant containing a bearing component made of UHMWPE. The method
includes the steps of radiation crosslinking a UHMWPE in the form of a
bulk material, preheating the crosslinked UHMWPE to a temperature above
80° C. and below its melting point, then solid state extruding the
preheated UHMWPE to a diametral compression ratio of greater than 1,
cooling the extruded UHMWPE to a solidification temperature below
30° C. while maintaining diametral compression, annealing the
cooled UHMWPE at a temperature below the melting point for a time
sufficient for the rod to increase in diameter in response to the
annealing, and machining the bearing component from the annealed UHMWPE.
The IJHMWPE is optionally sterilized after machining the bearing
component. Sterilizing is preferably performed by non-irradiative means
such as exposure to gases such as ethylene oxide.

[0030]In various embodiments, implants are manufactured using preformed
polymeric compositions having the structures described herein and made by
the methods described herein. Non-limiting examples of implants include
hip joints, knee joints, ankle joints, elbow joints, shoulder joints,
spine, temporo-mandibular joints, and finger joints. In hip joints, for
example, the preformed polymeric composition can be used to make the
acetabular cup or the insert or liner of the cup. In the knee joints, the
compositions can be made used to make the tibial plateau, the patellar
button, and trunnion or other bearing components depending on the design
of the joints. In the ankle joint, the compositions can be used to make
the talar surface and other bearing components. In the elbow joint, the
compositions can be used to make the radio-numeral or ulno-humeral joint
and other bearing components. In the shoulder joint, the compositions can
be used to make the glenero-humeral articulation and other bearing
components. In the spine, intervertebral disc replacements and facet
joint replacements may be made from the compositions.

[0031]In various embodiments, the bearing components are made from the
polymeric compositions by known methods such as by machining and are
incorporated into implants by conventional means.

Polymers

[0032]For implants, preferred polymers include those that are wear
resistant, have chemical resistance, resist oxidation, and are compatible
with physiological structures. In various embodiments, the polymers are
polyesters, polymethylmethacrylate, nylons or polyamides, polycarbonates,
and polyhydrocarbons such as polyethylene and polypropylene. High
molecular weight and ultra high molecular weight polymers are preferred
in various embodiments. Non-limiting examples include high molecular
weight polyethylene, ultra high molecular weight polyethylene (UHMWPE),
and ultra high molecular weight polypropylene. In various embodiments,
the polymers have molecular ranges from approximate molecular weight
range in the range from about 400,000 to about 10,000,000.

[0033]UHMWPE is used in joint replacements because it possesses a low
co-efficient of friction, high wear resistance, and compatibility with
body tissue. UHMWPE is available commercially as bar stock or blocks that
have been compression molded or ram extruded. Commercial examples include
the GUR series from Hoechst. A number of grades are commercially
available having molecular weights in the preferred range described
above.

Crosslinking

[0034]According to various embodiments of the invention, a crosslinked
polymeric bulk material is further processed in a series of heating,
deforming, cooling, and machining steps. The polymeric bulk material can
be crosslinked by a variety of chemical and radiation methods.

[0035]In various embodiments, chemical crosslinking is accomplished by
combining a polymeric material with a crosslinking chemical and
subjecting the mixture to temperature sufficient to cause crosslinking to
occur. In various embodiments, the chemical crosslinking is accomplished
by molding a polymeric material containing the crosslinking chemical. The
molding temperature is the temperature at which the polymer is molded. In
various embodiments, the molding temperature is at or above the melting
temperature of the polymer.

[0036]If the crosslinking chemical has a long half-life at the molding
temperature, it will decompose slowly, and the resulting free radicals
can diffuse in the polymer to form a homogeneous crosslinked network at
the molding temperature. Thus, the molding temperature is also preferably
high enough to allow the flow of the polymer to occur to distribute or
diffuse the crosslinking chemical and the resulting free radicals to form
the homogeneous network. For UHMWPE, a preferred molding temperature is
between about 130° C. and 220° C. with a molding time of
about 1 to 3 hours. In a non-limiting embodiment, the molding temperature
and time are 170° C. and 2 hours, respectively.

[0037]The crosslinking chemical may be any chemical that decomposes at the
molding temperature to form highly reactive intermediates, such as free
radicals, that react with the polymers to form a crosslinked network.
Examples of free radical generating chemicals include peroxides,
peresters, azo compounds, disulfides, dimethacrylates, tetrazenes, and
divinylbenzene. Examples of azo compounds are: azobis-isobutyronitrile,
azobis-isobutyronitrile, and dimethylazodi-isobutyrate. Examples of
peresters are t-butyl peracetate and t-butyl perbenzoate.

[0039]Generally, between 0.2 to 5.0 wt % of peroxide is used; more
preferably, the range is between 0.5 to 3.0 wt % of peroxide; and most
preferably, the range is between 0.6 to 2 wt %.

[0040]The peroxide can be dissolved in an inert solvent before being added
to the polymer powder. The inert solvent preferably evaporates before the
polymer is molded. Examples of such inert solvents are alcohol and
acetone.

[0041]For convenience, the reaction between the polymer and the
crosslinking chemical, such as peroxide, can generally be carried out at
molding pressures. Generally, the reactants are incubated at molding
temperature, between 1 to 3 hours, and more preferably, for about 2
hours.

[0042]The reaction mixture is preferably slowly heated to achieve the
molding temperature. After the incubation period, the crosslinked polymer
is preferably slowly cooled down to room temperature. For example, the
polymer may be left at room temperature and allowed to cool on its own.
Slow cooling allows the formation of a stable crystalline structure.

[0043]The reaction parameters for crosslinking polymers with peroxide, and
the choices of peroxides, can be determined by one skilled in the art.
For example, a wide variety of peroxides are available for reaction with
polyolefins, and investigations of their relative efficiencies have been
reported. Differences in decomposition rates are perhaps the main factor
in selecting a particular peroxide for an intended application.

[0044]Peroxide crosslinking of UHMWPE has also been reported. UHMWPE can
be crosslinked in the melt at 180° C. by means of
2,5-dimethyl-2,5-di-(tert-butylperoxy)-hexyne-3.

[0045]In various embodiments, crosslinking is accomplished by exposing a
polymeric bulk material to irradiation. Non-limiting examples of
irradiation for crosslinking the polymers include electron beam, x-ray,
and gamma-irradiation. In various embodiments, gamma irradiation is
preferred because the radiation readily penetrates the bulk material.
Electron beams can also be used to irradiate the bulk material. With
e-beam radiation, the penetration depth depends on the energy of the
electron beam, as is well known in the art.

[0046]For gamma (γ) irradiation, the polymeric bulk material is
irradiated in a solid state at a dose of about 0.01 to 100 Mrad (0.1 to
1000 kGy), preferably from 0.01 to 10 MRad, using methods known in the
art, such as exposure to gamma emissions from an isotope such as
60Co. In various embodiments, gamma irradiation is carried out at a
dose of 0.01 to 6, preferably about 1.5 to 6 Mrad. In a non-limiting
embodiment, irradiation is to a dose of approximately 5 MRad.

[0047]Irradiation of the polymeric bulk material is usually accomplished
in an inert atmosphere or vacuum. For example, the polymeric bulk
material may be packaged in an oxygen impermeable package during the
irradiation step. Inert gases, such as nitrogen, argon, and helium may
also be used. When vacuum is used, the packaged material may be subjected
to one or more cycles of flushing with an inert gas and applying the
vacuum to eliminate oxygen from the package. Examples of package
materials include metal foil pouches such as aluminum or Mylar®
coating packaging foil, which are available commercially for heat sealed
vacuum packaging. Irradiating the polymeric bulk material in an inert
atmosphere reduces the effect of oxidation and the accompanying chain
scission reactions that can occur during irradiation. Oxidation caused by
oxygen present in the atmosphere present in the irradiation is generally
limited to the surface of the polymeric material. In general, low levels
of surface oxidation can be tolerated, as the oxidized surface can be
removed during subsequent machining.

[0048]Irradiation such as γ-irradiation can be carried out on
polymeric material at specialized installations possessing suitable
irradiation equipment. When the irradiation is carmed out at a location
other than the one in which the further heating, compressing, cooling,
and machining operations are to be carried out, the irradiated bulk
material is conveniently left in the oxygen impermeable packaging during
shipment to the site for further operations.

Bulk Form of the Materials

[0049]The crosslinked polymer is provided in a bulk form characterized by
an axial direction and a transverse direction orthogonal or perpendicular
to the axial direction. In subsequent processing steps, deformation
pressure is applied on the crosslinked bulk material to reduce a
dimension in the transverse direction.

[0050]The axial direction is also the direction in which high tensile
strength is developed, as described further below. In this aspect, the
axial direction of the bulk material is the direction perpendicular to
the application of the deformation force that leads to development of
high tensile strength in the axial direction. In this way, application of
deformation pressure or force orthogonal to the axial direction creates
an anisotropic material, characterized by higher tensile strength in the
axial than in the transverse direction.

[0051]The axial direction of the bulk material also defines the preferred
direction in which implant bearing components such as acetabular cups are
to be machined. That is, bearing components are preferably made or
machined from the treated bulk polymer in an orientation where the high
tensile strength axis of the polymer corresponds to the load bearing axis
or direction of the bearing component of the implant in vivo.

[0052]In an exemplary embodiment, the bulk material is in the form of a
rod or cylinder having a circular cross section. The axial direction is
parallel to the main axis of the cylinder, while the transverse direction
is at right angles to the axial direction. In other words, the existence
of the axial direction defines an orthogonal direction referred to as
"transverse" in this application. When the cross section of the bulk
material is isotropic as in the case of a cylinder, the transverse
direction can be described as "radial", and the transverse axis as a
radial axis. The main axis of the bulk material can also be called the
longitudinal axis. As used here, the longitudinal axis is parallel to the
axial direction.

[0053]In the non-limiting case of a rod or cylinder, a cross section of
the bulk material perpendicular to the axial direction or longitudinal
axis is a circle. Other bulk materials characterized by an axial
direction may be used that have other perpendicular cross sections. In a
non-limiting example, a square cylinder can be provided that has a square
cross section perpendicular to the axial direction. Other bulk materials
characterized by an axial direction can have rectangular, polygonal,
star, lobed, and other cross sections perpendicular to the axial
direction.

[0054]In various embodiments, the axial direction of the bulk polymeric
material is elongated compared to the orthogonal or radial direction. For
example, in the case of UHMWPE, a commercially available bulk material is
a cylinder approximately 3 inches in diameter and 14 inches in length.
The length corresponds to the axial direction and the diameter
corresponds to the radial direction. As described below, bearing
components for implants are preferably machined from billets cut in the
axial direction. For efficiency in manufacturing it is convenient to
produce a number of bearing components from a single bulk material
treated by the methods of the invention. For this reason, the bulk
material is usually to be extended in an axial direction so as to be able
to cut a plurality of billets from the material for use in further
machining of the bearing components.

[0055]As described above, bulk material characterized by an axial
direction is further characterized as having a variety of cross sectional
areas perpendicular to the axial direction. In various embodiments, the
dimensions of the cross sectional areas perpendicular to the axial
direction are more or less constant along the axial direction from the
beginning to the end or from the top to the bottom of the bulk material.
In various other embodiments, bulk materials may be provided to have
cross sectional areas that vary along the length or axial direction of
the bulk material. In the case where the cross sectional area of the bulk
material is constant along the axial direction of the bulk material,
compressive force applied as described below will generally be applied to
the bulk material in a direction perpendicular to the axial direction. In
the case where the cross sectional area varies along the axial direction
of the bulk material, compressive force applied to the bulk material may
have a component in the axial direction due to the geometry of the bulk
material. However, in all cases at least a component of the compressive
force will be applied on the bulk material in a direction orthogonal to
the axial direction.

Pre-Heating

[0056]Before further processing, the crosslinked polymer is heated to a
compression deformable temperature. The compression deformable
temperature is temperature at which the polymeric bulk material softens
and can flow under the application of a compressive source to change
dimension in the direction the compressive force is applied. For UHMWPE
and other polymeric materials, the compression deformable temperature is
concretely from about the melting point minus 50° C. to the
melting point plus 80° C.

[0057]In various embodiments, the compression deformable temperature is
below the melting point of the polymeric material. Examples of the
compression deformable temperature include from the melting point to
10° C. below the melting point, from the melting point to
20° C. below the melting point, from the melting point to
30° C. below the melting point, and from the melting point to
40° C. below the melting point. For UHMWPE, the compression
deformable temperature is above 80° C., or from about 86°
C. to about 136° C., since the melting temperature of the UHMWPE
is about 136° C. to 139° C. In various embodiments, the
compression deformable temperature of UHMWPE lies from about 90°
C. to 135° C., preferably about 100° C. to 130° C. A
preferred temperature is 125-135° C., or 130°
C.±5° C.

[0058]In various embodiments, the crosslinked material is heated to a
compression deformable temperature above the melting point of the
polymer. For UHMWPE and other polymeric materials, such a compression
deformable temperature is from just above the melting point to a
temperature about 80° C. higher than the melting point. For
example, UHMWPE can be heated to a temperature of 160° C. to
220° C. or 180° C. to 200° C.

[0059]In various embodiments, it is preferred to heat the bulk polymeric
material to a compression deformation temperature close to but not higher
than the melting point. In various embodiments, the compression
deformable temperature is between the melting point and a temperature
20° C. lower than the melting point, or between the melting point
and a temperature 10° C. lower than the melting point.

[0060]The crosslinked bulk material can be heated to a compression
deformable temperature in a deformation chamber as illustrated in the
figures, or it can be preheated in an oven to the compression deformable
temperature. In various embodiments, the bulk material is heated to a
temperature just below the melting point, such as the melting point minus
5° or the melting point minus 10° and placed in a heated
deformation chamber. The deformation chamber preferably maintains a
compression deformable temperature. If desired, the deformation chamber
can be heated or thermostatted to maintain a constant temperature.
Alternatively, the deformation chamber is not itself heated but has
sufficient insulating properties to maintain the bulk material at a
compression deformable temperature during the course of extrusion through
the reducing die described below. In various embodiments, the temperature
of the deformation chamber is held at several degrees below the melting
temperature to avoid melting.

Deformation

[0061]When the crosslinked bulk material is at a compression deformable
temperature, deforming pressure is applied to the bulk material in a
direction orthogonal to the axial direction. The application of the
orthogonal force results in material flow of the heated bulk material. As
a result, a dimension of the bulk material in the transverse direction at
which force is applied is diminished compared to the original dimension.
As discussed above, compression force is applied so that least one
component of the force is orthogonal to the axial direction of the bulk
material. For cylindrical rods and other bulk materials that have a
constant cross section along the axial direction of the bulk material,
the compression force is applied in a direction perpendicular to the
axial direction.

[0062]Any suitable methods may be used to apply the compression force in a
direction orthogonal to the axial direction. Non-limiting examples
include rollers, clamps, and equivalent means.

Extrusion

[0063]In various embodiments, deforming force is applied in the
directional orthogonal to the axial direction of the bulk material by
extruding the bulk material through a reducing die. Pressure exerted on
the bulk material in a direction orthogonal to the axial direction during
extrusion causes the dimension of the bulk material to be reduced
compared to the original dimension of the bulk material. In other words,
the diameter or other transverse dimension of the bulk material after
extrusion is less than the dimension before extrusion.

[0064]The relative reduction in the dimension of the bulk material in the
transverse directions can be expressed as a ratio of the original
dimension d1 to the reduced dimension d2. Depending on the
method of reducing the dimension by applying compressive force, the
numeric value of the ratio d1/d2 can be referred to as a draw
ratio or a diametral compression. For extrusion, it is common practice to
refer to a draw ratio; unless stated otherwise from context, the term
draw ratio will be used here to refer to all geometries.

[0065]It is to be understood that the transverse direction (the direction
orthogonal to the axial direction) in which deformation pressure or force
is applied itself contains two axes that can be drawn at right angles to
the longitudinal axis. In various embodiments, the bulk material can be
deformed by a different amount along the two transverse axes, and a draw
ratio can be defined for both axes. The orientation of the transverse
axes is arbitrary; if needed for analysis, the axes can be selected to
simplify the geometry of the applied forces. When the cross section of
the bulk material is isotropic, equal deformation force can be applied in
all transverse directions. In this non-limiting case, the dimension
d2 corresponds to the radius or diameter of the extruded material,
and the draw ratio is the fraction defined by dividing d1 by
d2.

[0066]In various embodiments, the draw ratio is 1.1 or higher, and less
than about 3. In various embodiments, the draw ratio is 1.2 or higher,
and is preferably about 1.2 to 1.8. It is about 1.5 in a non-limiting
example. At high levels of reduction, a point is reached at which the
strain introduced is too great and the properties of the crosslinked
polymeric materials deteriorate. Accordingly, in various embodiments the
draw ratio is 2.5 or less, and preferably about 2.0 or less. In a
preferred embodiment, the compressive force is applied more or less
isotropically around the bulk material in a direction transverse to a
longitudinal axis. Accordingly, the reduction in dimension will usually
apply in all transverse directions. To illustrate, a circular cross
section remains round but is reduced in diameter, while a polygonal cross
section such as a square or rectangle is reduced on all sides.

[0067]The geometry of extrusion through a reducing die is illustrated in
schematic form in FIGS. 1 and 2. A reducing die 6 is disposed between a
deformation chamber 2 and a cooling chamber 4. As shown, the reducing 6
die serves to reduce the diameter or dimension of the extruded rod from
an original dimension d1 to an extruded dimension d2. As the
crosslinked heated bulk material passes from the deformation chamber
through the reducing die 6, the material flows by the die wall 5 that
leads to a constriction 10 having the diameter d2 of the cooling
chamber 4.

[0068]Various geometries of the reducing die are illustrated in
non-limiting form in FIG. 2. FIGS. 2a to 2e show the relative
configuration of the deformation chamber wall 20 and the cooling chamber
wall 10. The die wall 5 is seen to connect the cooling deformation
chamber to the cooling chamber. In FIG. 2a, the cross section of both the
deformation chamber 2 and cooling chamber 4 are circular, with dimensions
d1 and d2 corresponding to their respective diameters. In FIG.
2b, the deformation chamber is square or rectangular characterized by a
dimension d1 that can be arbitrarily taken along a diagonal or along
a side. In FIG. 2b, the cooling chamber 4 is also rectangular but having
lower dimension d2. FIGS. 2c through 2e illustrate other
combinations of circular, square, and triangular deformations and cooling
chambers connected by reducing dies 6 having a die wall 5, and are
offered by way of non-limiting example.

[0069]As noted above, the bulk material in the deformation chamber 2 is
held at a compression deformable temperature. At such a temperature, the
material can flow in response to pressure exerted on the material. When
the compression deformable temperature is below the melting point, the
material undergoes a solid state flow through the reducing die 6.
Pressure or force applied to the end of the bar by the ram is translated
by the die into compressive force that reduces the dimension of the bulk
material in the transverse direction. Conveniently, the diameter of the
bulk material to be extruded matches relatively closely the diameter or
dimension d1 of the deformation chamber illustrated in FIG. 1.

Cooling

[0070]In various embodiments, an extruded UHMWPE rod or other crosslinked
polymeric material in a bulk form characterized by an axial direction is
cooled before further processing. Alternatively, the extruded bulk
material can be directly processed by the stress relief step described
below. In a non-limiting embodiment, the rod or other bulk material
characterized by an axial direction is cooled to a solidification
temperature in a cooling chamber or other means while pressure is
maintained sufficient to keep the dimension of the extruded bulk material
below the original dimension of the crosslinked bulk material. In the
extrusion or other compressive force embodiments, the pressure required
to maintain the dimension lower than the original dimension may be more
or less pressure than required to originally change the shape of the
polymer, such as through extrusion. As noted, the bulk material such as
extruded UHMWPE is held in a cooling chamber or similar device for a
sufficient time to reach a temperature at which the bulk material no
longer has a tendency to increase in dimension upon removal of the
pressure. This temperature is designated as the solidification
temperature; for UHMWPE the solidification temperature is reached when a
thermostat embedded in the cooling wall (about 1 mm from the inside wall
surface) reads about 30° C. The solidification temperature is not
a phase change temperature such as a melting or freezing. It is also to
be noted that a material such as UHMWPE can be cooled to the
solidification temperature independently of whether the material was
heated above or below the melting point in a previous processing step.

[0071]In various embodiments, after extrusion or other application of
deforming force in a direction orthogonal to the axial direction of the
crosslinked polymeric bulk material, the compressive deforming force is
maintained on the bulk material until the bulk material cools to the
solidification temperature. Such a maintenance of compressive force is
conveniently provided in the reducing die embodiment illustrated in FIGS.
1 and 2. After extrusion through the reducing die 6, the bulk material is
held in the cooling chamber 4. In the embodiment shown in the Figures,
the cooling chamber is of such a size and shape as to hold the extruded
bulk material at a dimension or diameter d3, which is less than the
original dimension d1 of the bulk material and is conveniently about
the same as the extruded dimension d2 in a non-limiting example. The
crosslinked material has a tendency to return to its original dimension
by expanding when the temperature is above the solidification
temperature. The expansion force of the bulk material is counteracted by
the walls of the cooling chamber, with the result that compressive force
is maintained on the bulk material while it cools. In various
embodiments, the cooling chamber is provided with cooling means such as
cooling jackets or coils to remove heat from the cooling chamber and the
extruded polymer bulk material.

[0072]Referring to the figures for illustration, as the polymeric extruded
bulk material cools in the cooling chamber, a temperature is reached at
which the material no longer has a tendency to expand or revert to its
original dimension d1. At this temperature, called the
solidification temperature, the bulk material no longer exerts pressure
on the walls of the cooling chamber and can be removed. In preferred
embodiments, the material is cooled to about 30° C., as measured
by thermostats in the walls of the chamber, before removal.

[0073]The temperatures of the deformation chamber and the cooling chamber
can be measured by conventional means, such as by thermocouples embedded
into the walls of the respective chambers. For example, it has been found
that when a thermocouple in the wall of the cooling chamber indicates a
temperature of 30° C., an extruded bulk material made of UHMWPE
has reached a bulk temperature below a solidification temperature at
which the material loses it tendency to expand. The temperature as
measured with, for example, a thermocouple embedded in the wall of the
cooling chamber does not necessarily represent a bulk or equilibrium
temperature of the material in the cooling chamber. An appropriate rate
of cooling may be provided in the cooling chamber by use of heat exchange
fluids such as water or water glycol mixture, and the bulk material held
in the cooling chamber for a time and until a temperature is reached at
which it is observed that removal of the bulk material from the chamber
does not result in significant increase in diameter. Thus, in various
embodiments, cooling to a solidification temperature of, for example,
90° F. or 30° C. means leaving the extruded bulk material
in the cooling chamber until the thermocouple embedded in the walls of
the cooling chamber reads 90° F. or 30° C. As noted, it has
been found that such a cooling period suffices for removal of the bulk
material, even though the bulk equilibrium temperature of the interior of
the bulk material could be higher than the measured temperature.

[0074]In various embodiments, the extruded bulk material is held in the
cooling chamber for an additional period of time, such as 10 minutes,
after the embedded thermocouple reads 90° F. or 30° C. The
additional cooling period can enable the cooled material to be more
easily removed from the cooling chamber. In one embodiment, when the
thermocouple reaches a reading of 30° C., a programmable logic
controller (plc) starts a timer that in turns gives a signal when the
desired time has passed. At that time an operator can remove the
compression deformed crosslinked material from the chamber, or rams or
other suitable devices can be actuated to effect removal.

Sacrificial Puck

[0075]In a preferred embodiment, a so-called sacrificial puck is used to
improve the efficiency of the extrusion process. In referring to FIG. 3,
a ram 30 is provided in a retracted position with respect to the
deformation chamber 2. FIG. 3b shows the ram 30 retracted and the
deformation chamber 2 filled with a rod-like bulk material 50 and a
sacrificial puck 40. The sacrificial puck 40 is made of a crosslinked
polymer, which may be the same as the crosslinked polymer of the bulk
material 50. It is preferably of approximately the same cross-sectional
shape and area as the bulk material 50 to be extruded. In FIG. 3c, the
ram 30 is shown pushing on the sacrificial puck 40, which in turn pushes
on the bulk material 50 to move the bulk material 50 through the reducing
die 6 into the cooling chamber 4. FIG. 3d shows the situation at the end
of the stroke of the ram 30. The bulk material 50 is sitting completely
in the cooling chamber 4, while the sacrificial puck 30 occupies the
reducing die 6. Upon retraction of the ram 30 as shown in FIG. 3e, the
sacrificial puck 40 tends to return to its original dimension because it
is not being cooled in the cooling chamber as the bulk material 50 is. As
a result, the sacrificial puck tends to extricate itself from the
reducing die as shown in FIG. 3f. The sacrificial puck 40 can then be
removed from the deformation chamber and the process repeated after a
cycle time in which the bulk material 50 cools to a suitable
solidification temperature as discussed above.

Stress Relieving

[0076]Following extrusion and optional cooling to a solidification
temperature, the bulk material is then preferably stress relieved. In one
embodiment, stress relieving is carried out by heating to a stress relief
temperature, preferably below the melting point of the polymeric bulk
material. If the cooling in the previous step is carried out while
maintaining deformation force, the bulk material on stress relieving
tends to expand and return to a dimension close to its original
dimension. In the non-limiting example of an extruded rod, as the bulk
material is heated, the diameter d3 of the rod tends to increase to
a diameter approaching d1 of the original bulk material. In various
non-limiting embodiments, it has been observed that the bulk material
retains about 90-95% of its original dimension upon stress relieving or
stress relief heating.

[0077]The stress relief process tends to run faster and more efficiently
at higher temperatures. Accordingly, stress relief temperatures close to
but less than the melting temperature are preferred, for example from the
melting point to the melting point minus 30 or 40° C. For UHMWPE,
preferred stress relief temperatures include in the range of about
100° C. to about 135° C., 110° C. to about
135° C., 120° C. to 135° C., and preferably
125° C. to about 135° C.

[0078]Stress relieving is carried out for a time to complete the stress
relief process. In various embodiments, suitable times range from a few
minutes to a few hours. Non-limiting examples include 1 to 12 hours, 2 to
10 hours, and 2 to 6 hours in an oven or other suitable means for
maintaining a stress relief temperature. Although the stress relieving
can be carried out in a vacuum, in an inert atmosphere, or in a package
designed to exclude an atmosphere, it is preferably carried out in an air
atmosphere.

[0079]Under some conditions, the solidified extruded bulk form exhibits a
tendency to bend or other deviate from a preferred straight or linear
orientation during the heating or other treatment associated with stress
relieving. To counter this tendency, in one embodiment, the bulk material
is held in a mechanical device that functions to keep the bulk material
straight (measured on the axial direction) during the stress relieving
step. In a non-limiting example, the bulk material is placed into
V-channels to keep them straight. For example, several V-channels are
equally spaced from each other and are part of the same physical
structure. The several V-channels may, for example, be welded to the
structure at equal spacings. The extruded bars are positioned on a bottom
set of V-channels and then another set of V-channels is set on top of the
extruded bars to rest on top of the bars. These channels help to keep the
bars straight during stress relieving.

[0080]In various embodiments, the product of the crosslinking, heating,
compressing, cooling and stress relieving steps is a bulk material having
dimensions approximately equal to the original bulk material before
crosslinking. As a result of the steps taken on the bulk material, the
bulk material exhibits high tensile strength in the axial direction, a
low but detectable level of free radical concentration, and a high degree
of resistance to oxidation.

[0081]The process described can be followed with regard to the dimensions
of the crosslinked polymer at various stages of the process. In various
embodiments, a bulk material having an original dimension or diameter of
d1 is crosslinked and heated to a compression deformation
temperature. The crosslinked heated material is then compressed to a
dimension or diameter d2 which is less than d1. In an optional
step, the material is then held while cooling at a diameter d3 that
may be the same as d2, but in any case is less than the original
dimension or diameter d1. After cooling, stress relieving returns
the bulk material to a diameter d4 which is greater than d3 and
in some embodiments is approximately equal to the original dimension or
diameter d1. For example, if the original bulk material is a
3''×14'' cylinder of UHMWPE, the treated preform resulting from the
steps above preferably typically has a diameter of about 2.7 to 3 inches.

[0082]Following the treatment steps described above, the bulk material
characterized by an axial direction is machined according to known
methods to provide bearing components for implants. In the case of a
cylindrical treated bulk material perform, it is preferred first to turn
the outer diameter of the cylinder to remove any oxidized outer layers
and to provide a straight and round cylinder for further processing. In a
preferred embodiment, the cylinder is then cut into billets along the
axial direction, and each billet is machined into a suitable bearing
component. Preferably, the bearing components are machined from the
billets in such a way that the in vivo load bearing axis of the bearing
component corresponds to the axial direction of the bulk preform from
which it is machined. Machining this way takes advantage of the increased
tensile strength and other physical properties in the axial direction of
the preform.

[0083]For example, in bearing components for joint replacements, the
stresses at the bearing surface are typically multiaxial, and the
magnitude of the stresses further depends on the conformity of the joint.
For hip applications, the polar axis of the cup is aligned with the
longitudinal axis of the extruded rod, corresponding to the axial
direction. The wall of the cup, at the equator and rim, is parallel to
the long axis of the rod, and will benefit from the enhanced strength in
this direction during eccentric and rim loading scenarios.

Oxidative Resistance

[0084]It has been found that UHMWPE, preforms, and bearing components made
according to the invention have a high level of oxidative resistance,
even though free radicals can be detected in the bulk material. To
measure and quantify oxidative resistance of polymeric materials, it is
common in the art to determine an oxidation index by infrared methods
such as those based on ASTM F 2102-01. In the ASTM method, an oxidation
peak area is integrated below the carbonyl peak between 1650 cm-1
and 1850 cm-1. The oxidation peak area is then normalized using the
integrated area below the methane stretch between 1330 cm-1 and 1396
cm-1. Oxidation index is calculated by dividing the oxidation peak
area by the normalization peak area. The normalization peak area accounts
for variations due to the thickness of the sample and the like. Oxidative
stability can then be expressed by a change in oxidation index upon
accelerated aging. Alternatively, stability can be expressed as the value
of oxidation attained after a certain exposure, since the oxidation index
at the beginning of exposure is close to zero. In various embodiments,
the oxidation index of crosslinked polymers of the invention changes by
less than 0.5 after exposure at 70° C. to five atmospheres oxygen
for four days. In preferred embodiments, the oxidation index shows a
change of 0.2 or less, or shows essentially no change upon exposure to
five atmospheres oxygen for four days. In a non-limiting example, the
oxidation index reaches a value no higher than 1.0, preferably no higher
than about 0.5, after two weeks of exposure to 5 atm oxygen at 70°
C. In a preferred embodiment, the oxidation index attains a value no
higher than 0.2 after two or after four weeks exposure at 70° to 5
atm oxygen, and preferably no higher than 0.1. In a particularly
preferred embodiment, the specimen shows essentially no oxidation in the
infrared spectrum (i.e. no development of carbonyl bands) during a two
week or four week exposure. In interpreting the oxidative stability of
UHMWPE prepared by these methods, it is to be kept in mind that the
background noise or starting value in the oxidation index determination
is sometimes on the order of 0.1 or 0.2, which may reflect background
noise or a slight amount of oxidation in the starting material.

[0085]Oxidation stability such as discussed above is achieved in various
embodiments despite the presence of a detectable level of free radicals
in the crosslinked polymeric material. In various embodiments, the free
radical concentration is above the ESR detection limit of about
0.06×1015 spins/g and is less than that in a gamma sterilized
UHMWPE that is not subject to any subsequent heat treatment (after
sterilization) to reduce the free radical concentration. In various
embodiments, the free radical concentration is less that
3×105, preferably less 1.5×1015, and more
preferably less than 1.0×1015 spins/g. In various embodiments,
the oxidation stability is comparable to that of melt processed UHMWPE,
even if according to the invention the UHMWPE is processed only below the
melting point.

[0086]Although the invention is not to be limited by theory, the free
radicals in the deformation processed UHMWPE described above may be
highly stabilized and inherently resistant to oxidative degradation.
Alternatively or in addition, they may be trapped within crystalline
regions of the bulk material and as a consequence may be unavailable to
participate in the oxidation process. Because of the oxidation stability
of the material, in various embodiments it is justifiable to employ gas
permeable packaging and gas plasma sterilization for the extrusion
processed radiation UHMWPE. This has the advantage of avoiding gamma
sterilization, which would tend to increase the free radical
concentration and lead to lower oxidation stability.

[0087]In various embodiments, the solid state deformation process provides
polymers that are characterized by a crystal and molecular orientation.
By molecular orientation is meant that polymer chains are oriented
perpendicular to the direction of compression. By crystalline orientation
it is meant that crystal planes in polyethylene, such as the 200 plane
and the 110 plane are oriented to the direction parallel to the
compression plane. In this way the crystal planes are oriented. The
presence of the orientations can be shown by means of birefringent
measurements, infrared spectra, and x-ray diffraction.

[0088]The plane of compression for articles compressed in a radial
direction is understood to be a surface surrounding and parallel to the
radial surface of the bulk material that is processed according to the
invention. In the non-limiting example of a cylindrical rod, a sequence
of circular cross sections along the axial direction defines a radial
surface and a compression plane perpendicular to that surface. In
response to compression around the radial plane, polymer chains orient
themselves perpendicular to the direction of compression. This has the
effect in a cylinder of providing molecular orientation generally
parallel to the radial plane. It is believed that with this molecular and
crystal orientation contributes to the enhancement of mechanical
properties, and to anisotropy in the mechanical properties with respect
to the axial and transverse (or radial) directions.

[0089]In various embodiments, crosslinked UHMWPE are provided that exhibit
a high level of tensile strength in at least one direction.
Advantageously, bearing components and implants are provided that take
advantage of the increased strength of the bearing material. For example,
in crosslinked UHMWPE, it is possible to achieve a tensile strength at
break of at least 50 MPa, preferably at least 55 MPa, and more preferably
at least 60 MPa. In various embodiments, materials are provided with a
tensile strength at break in the range of 50-100 MPa, 55-100 MPa, 60-100
MPa, 50-90 MPa, 50-80 MPa, 50-70 MPa, 55-90 MPa, 55-80 MPa, 55-70 MPa,
60-90 MPa, 60-80 MPa, and 60-70 MPa. In a non-limiting embodiment the
tensile strength of a UHMWPE prepared according the invention is about 64
MPa in the axial direction.

EXAMPLES

Comparative Example

[0090]Isostatically molded UHMWPE bar stock (Ticona, Inc., Bishop, Tex.)
is packaged in an argon environment and gamma sterilized to a dose of 25
to 40 kGy

Example 1

[0091]Radiation crosslinked, deformation processed UHMWPE is produced
using the following steps:

[0093]2. Preheating. Prior to deformation processing, the rod is removed
from the foilized bag and raised to 133° C. for 4 to 12 hours in
an oven.

[0094]3. Solid state, hydrostatic extrusion. The heated rod is then
removed from the oven and placed in the holding chamber of a press. The
temperature of the holding chamber is 130° C.±5° C. The
bar is then ram extruded using a sacrificial puck made of crosslinked
UHMWPE through a circular die, into a cooling chamber with a diametral
compression ratio of 1.5 (diameter of 3'' down to 2'').

[0095]4. Cooling and solidification. The cooling chamber is sized so as to
maintain the extruded rod in a deformed state. The walls of the cooling
chamber are water-cooled. When thermocouples embedded in the wall (about
1 mm from the inside wall) read 30° C., the solidified rod is
removed, optionally after an additional cooling period of ten minutes, in
a non-limiting example. If desired, a second bar is ram extruded to eject
the cooled bar from the cooling chamber, once the temperature reaches
about 30° C.

[0096]5. Stress relief, annealing. The deformed rod is then heated at
133±2° C. for 5 hours. The annealing also improves dimensional
stability in the material. The rod is then slowly cooled to room
temperature. The extruded rod retains about 90-95% of its initial
diameter after the stress relief step.

[0097]6. Gas plasma sterilization. After cooling, a liner or other bearing
material is machined and the machined part is non-irradiatively
sterilized (e.g., with ethylene oxide or gas plasma)

Specimen Preparation and Orientation

[0098]For compression tests and accelerated aging, right rectangular prism
specimens are evaluated. The specimens measure 12.7 mm by 12.7 mm by 25.4
mm (0.50 in. by 0.50 in. by 1.00 in.) They are machined from the rod
stock parallel (the axial direction) or perpendicular (the transverse
direction) to the long axis.

[0099]For tensile tests, dumbbell-shaped tensile specimens consistent with
the Type IV and V specimen description provided in ASTM D638-02a are
tested. Specimens are 3.2±0.1 mm thick. Specimens are oriented
parallel or perpendicular to the long axis, reflecting the axial and
transverse directions, respectively).

Physical and Mechanical Properties

[0100]Tensile strength at break is determined according to ASTM 638-02a.

[0101]The concentration of free radicals in the UHMWPE materials is
characterized using an ESR spectrometer (Bruker EMX), as described
previously in Jahan et al., J. Biomedical Materials Research, 1991; Vol.
25, pp 1005-1017. The spectrometer operates at 9.8 GHz (X Band) microwave
frequency and 100 kHz modulation/detection frequency, and is fitted with
a high sensitivity resonator cavity. For a good spectral resolution
and/or signal-to-noise ratio, modulation amplitude is varied between 0.5
and 5.0 Gauss, and microwave power between 0.5 and 2.0 mW.

Accelerated Aging

[0102]Specimens are aged in 5 atmospheres of oxygen in accordance with
ASTM F 2003-00. Some specimens are aged for two weeks according to this
standard, and others are aged for four weeks. Aging is performed in a
stainless steel pressure vessel. The specimens are chosen and oriented
such that the tested axis is vertical. Thus, the top and bottom faces are
perpendicular to the test axis. The top face is labeled for later
identification. The vessels are then filled with oxygen and purged five
times to ensure the purity of the aging environment. The prisms rest on a
flat surface inside the pressure vessel; thus each prism's bottom face is
not exposed to oxygen, but each of its other faces are exposed to oxygen
throughout the aging period.

[0103]The vessel is placed in the oven at room temperature
(24±2° C.), and the oven was heated to the aging temperature of
70.0±10.1° C. at a rate of 0.1° C./min.

FTIR Analysis

[0104]Materials are evaluated before and after accelerated aging by
Fourier transform infrared spectroscopy (FTIR) in transmission (Excalibur
series FTS3000 with a UMA-500 microscope attachment; Bio-Rad
Laboratories, Hercules, Calif.). FTIR profiling is conducted
perpendicular to the transverse direction.

[0105]Oxidation index measurement and calculations are based on ASTM F
2102-01. Oxidation peak area is the integrated area below the carbonyl
peak between 1650 and 1850 cm-1. The normalization peak area is the
integrated area below the methylene stretch between 1330 and 1396
cm-1. Oxidation index is calculated by dividing the oxidation peak
area by the normalization peak area.

Results

[0106]Data for the Comparative Example and Example 1 are given in the
Table

[0107]Although the invention has been described above with respect to
various embodiments, including those believed the most advantageous for
carrying out the invention, it is to be understood that the invention is
not limited to the disclosed embodiments. Variations and modifications
that will occur to one of skill in the art upon reading the specification
are also within the scope of the invention, which is defined in the
appended claims.